1. Field of the Invention
This invention relates to a phosphating solution and to a process for phosphating surfaces of steel, zinc, aluminium or their alloys. It is particularly suitable for phosphating electrolytically galvanized or hot-dip-galvanized steel. After the intended contact time on the surfaces, the phosphating solutions are not rinsed off with water, but instead are immediately dried on the lines of a so-called no-rinse process. Accordingly, the process according to the invention is particularly suitable for use in continuous strip treatment plants.
The object of phosphating metals is to produce on the metal surface firmly intergrown metal phosphate coatings which improve resistance to corrosion and, in conjunction with paints and other organic coatings, lead to a significant increase in paint adhesion and in resistance to creepage in corrosive environments. Phosphating processes have long been known. Recently, low-zinc phosphating processes where the phosphating solutions have comparatively low contents of zinc ions, for example of 0.5 to 2 g/l, have been particularly preferred for pretreatment in preparation for painting, more particularly electrolytic dip coating as normally applied in the automotive industry.
In the automotive industry and, in particular, in the domestic appliance industry, but also for architectural applications, there has been a tendency to use galvanized steel strip pre-phosphated in the steel mill in order to utilize the more favorable forming properties of phosphate-coated strip and to save on chemical treatment steps before painting. Accordingly, increasing importance is being attached to phosphating processes which lead to high-quality phosphate coatings despite the short phosphating times of the strip mill of only a few seconds. The treatment is normally carried out by spraying, by immersion or by combined spraying/immersion, the phosphating solution being rinsed off from the metal surface with water after the required contact time. One such process is described, for example, in DE-A-42 41 134, according to which phosphating solutions containing 1.0 to 6.0 gl of zinc and 8 to 25 g/l of phosphate are used. Other optional components are nickel, cobalt, manganese, magnesium and calcium each in quantities of 0.5 to 5.0 g/l, iron(II) in quaintities of up to 2 g/l and copper in quantities of 3 to 50 mg/l.
2. Related Art
The hitherto necessary removal of the phosphating solution by rinsing off with water leads on the one hand to a high consumption of fresh water in the phosphating plant and, on the other hand, results in the accumulation of wastewater contaminated with heavy metals which has to be treated for reuse or for discharge into the main drains. The concept of no-rinse phosphating has already been discussed in the literature (G. Carreras-Candi: "Characteristiques de la Phosphatation sans Rincage" . . . , Surfaces 106 (1976), Number 15, pages 25-28) without any concrete information on how the process is carried out or on suitable treatment baths.
DE-C-27 39 006 describes a phosphating process which eliminates the need for rinsing with water which is undesirable from the environmental and cost point of view. In this process, the surfaces are contacted for 1 to 5 seconds at 50 to 75.degree. C. with a phosphating solution which contains 0.1 to 5 g/l of zinc, 1 to 10 parts by weight of nickel and/or cobalt per part by weight of zinc, 5 to 50 g/l of phosphate and--as accelerator--0.5 to 5 g/l of hydrogen peroxide. The surfaces are immediately dried without rinsing. The use of phosphating solutions containing more than 5 g/l of zinc is discouraged because they have an adverse effect on paint adhesion.
EP-B-141 341 also describes a no-rinse phosphating process. This process was developed in particular for fixed structures, such as bridges or the like. Accordingly, the surfaces to be protected are treated with a solution containing 1 to 5% by weight of zinc, 1 to 20% by weight of phosphoric acid, 0.01 to 0.5% by weight of cobalt and/or nickel and 0.02 to 1.5% by weight of an accelerator. After application of the phosphating solution, for example by wiping, brushing, spread coating, roll coating or spray coating, the solution is left to act for an unspecified time, the solution either reacting out or only partly reacting. In both cases, the surfaces can be rinsed after exposure to the phosphating solution.
In contrast to conventional phosphating processes which are used in preparation for painting, the phosphating solution described above has high zinc and phosphate contents. Phosphating solutions in similar concentration ranges are also known for the deposition of phosphate coatings onto metal parts which are to be subjected to cold mechanical forming, for example by drawing or pressing. The relatively thick phosphate coatings deposited, which may be impregnated with oil to enhance their effect, act as lubricants and reduce friction between tool and workpiece. They are not normally suitable as a pretreatment before painting because paint adhesion to the thick phosphate coatings under mechanical stressing is very poor. A corresponding phosphating solution which may be used to form phosphate coatings on steel strip or steel wire, acting as a lubricant before cold drawing or other forming processes, is described for example in DE-B-25 52 122. According to this document, the solutions used contain zinc in a quantity of 5 to 100 g/l and phosphate in a quantity of 10 to 150 g/l and--as accelerator--nitrate in a quantity of 10 to 80 g/l. The phosphating solution is brought into contact with the surface for 5 to 15 seconds and then rinsed off with water.
The problem addressed by the present invention was to provide phosphating processes and phosphating solutions which are intended for use in continuous strip mills and which eliminate the need to rinse the treated surfaces with water.